Linear power amplifiers that can maintain high efficiencies over wide ranges of output power levels are key components in modern communication systems. Achieving high efficiencies at back-off power levels is important both in base and mobile stations since the former must handle multi-carrier signals with high peak-to-average power ratios (high PARs) and the latter often should operate at low powers to maximize the battery lifetime. On the other hand, these amplifiers must meet stringent linearity requirements to keep the out of band spectral emissions within the Federal Communications Commission (FCC) recommended spectral masks. In addition, with the growth of the quantity of data transfer the amplifier also has to support wider bandwidth.
To fulfill the first two requirements, several techniques have been proposed with two distinct approaches to the problem. Techniques such as envelope elimination and restoration (EER), polar transmitters, and linear amplification using nonlinear components (LINC) improve the linearity of high-efficiency nonlinear amplifiers by driving them with constant-envelope signal(s). Other approach uses bias adaptation or load modulation techniques to improve the efficiency of linear amplifiers.
As disclosed in U.S. Pat. No. 2,210,028, Doherty power amplifiers (DPAs) have demonstrated high efficiencies over wide ranges of output power levels. They are simple, easy to implement and relatively wideband as compared to other efficiency enhancement techniques.
A classic Doherty amplifier consists of two amplifiers a main amplifier and an auxiliary amplifier (see FIG. 1A and its corresponding graphs in FIGS. 1B-1E). The main amplifier is used as a carrier amplifier and biased to operate in Class AB mode, where the auxiliary amplifier is used as a peaking amplifier biased to operate in Class C mode. A power divider splits the input signal equally to each amplifier with 90 degrees difference in phase. After amplification, the signals are recombined with a power combiner. When the amplifier's drive level is less than a specific value, only the Class AB carrier amplifier provides amplification and is presented with load impedance that produces high efficiency and gain. The role of the auxiliary cell is to actively modulate the main amplifier's load impedance while contributing to the output power at the same time.
When the input signal peaks (as is the case with high-PAR signals), the Class C peaking amplifier also begins to deliver amplification to handle the highest power output levels, and produces a load impedance that allows both amplifiers to provide the highest possible output power. The Doherty amplifiers can achieve high efficiencies as well as linear characteristics provided that the perfect load modulation scheme is realized.
It is well known, that the key action of the Doherty PA (DPA) occurs in the high power region where the auxiliary amplifier is activated and the main amplifier is held at the maximum voltage as shown in FIG. 1B. This is achieved through the dynamic load modulation of the main amplifier due to the load-pulling effect provided by the auxiliary amplifier. Due to the voltage-saturated operation of the main amplifier, the overall efficiency of the DPA is significantly improved as shown in FIG. 1C. This figure also shows that DPAs are ideally linear amplifiers. During the high power mode, the auxiliary amplifier's contribution to the output power compensates the square root power transfer function of the main amplifier to realize a linear input-output power characteristic. The DPAs linearity can also be studied from the intermodulation (IM) products point of view.
In the high power region, the two amplifiers generate IM products with 180 deg phase difference because the main amplifier has gain compression while the auxiliary one experiences gain expansion. Consequently, the IM products cancel out each other, leaving the DPA with a distortion-free characteristic. Yet, in practice, when a transistor is being used, the main amplifier do not obey to a constant saturation level at Vo max (unless it was reaching its nonlinear level much before), and therefore the overall output power do not act like a linear curve. A gain and phase compression occurs at the moment that the auxiliary amplifier begins to deliver power, and the IM products level is not as low as in theory.
However, a Doherty PA produces less linearity and RF output power than a Class AB amplifier. Moreover, the Doherty PA that is based on transistors such as FET's can truly provide a superior improvement of efficiency, but with the cost of some degradation in linearity performances. To have perfect linearity, the Doherty PA theory assumes that during the activation of the auxiliary amplifier the main amplifier reached already its saturation level. In practice, that is not the case due to the FET typical behavior. Furthermore, if it was the case then the linearity of the Doherty PA would have been degrade even before the auxiliary “on” state interval. Well-known design techniques such as adaptive bias, switched Doherty, and digital predistortion have been proposed for performance improvement in Doherty amplifiers.
Another well-known standalone efficiency-enhancement technique for DPA is the switching adaptive biasing as disclosed in U.S. Pat. No. 6,437,641 (known as XNN®-Based Power Amplifier Booster by Tower Semiconductor and Paragon Communications), which is used to get an improvement of efficiency by introducing a higher drain voltage to the FET when a peak level is reached. However, this method is limited due to the change of the transistor impedance in function of the Drain voltage level.
Therefore, in order to solve both methods limitations it is offered to use a novel approach that refers herein as an “Adaptive Impedance” method.
It is an object of the present invention to provide a power amplifier system that involve a main amplifier and one or more auxiliary amplifiers, which is capable of changing the impedance of the main amplifier adaptively, in such a way, that its new impedance in parallel to the new impedance of the auxiliary amplifier will be similar to the impedance of the auxiliary amplifier when it was inactive (i.e., at “off” state).
Other objects and advantages of the invention will become apparent as the description proceeds.